(a) Field of the Invention:
This invention relates to an illuminating optical system adapted to endoscopes particularly of a high magnification.
(b) Description of the prior art:
In order to well observe the structure within a human body, it is necessary to use an endoscope high in the magnification. Therefore, hitherto it has been usual to use an endoscope of a type provided with an illuminating optical system on the side of an observing optical system as set very close to an object to be observed as shown in FIG. 1. That is to say, in FIG. 1, the reference symbol A.sub.1 shows a range to be illuminated on a surface 3 of an object to be observed of an illuminating light projected out of light guide fibers 1 through an illuminating optical system 2 and A.sub.2 shows a range to be observed on the surface 3 of the object to be observed of an observing optical system including a cover glass 4, objective 5 and image guide 6. In such formation, there have been defects that, in case the surface 3 of the object to be observed and the tip part 7 of the endoscope are made to approach to each other, the range A.sub.1 to be illuminated and range A.sub.2 to be observed will not coincide with each other and the uneven illumination will be made due to a parallax. There have been also defects that, incase the magnification of the observing optical system is high, the focal depth will be so shallow that, if the distance between the surface 3 of the object to be observed and the tip surface of the endoscope varies, the image will be likely to be out of focus and, if the tip part 7 of the endoscope deviates, in the case of photographing, the image will be likely to be fluid.
In order to eliminate such defects, such improvements as are shown in FIGS. 2 and 3 have been already made. First shown in FIG. 2 is of a structure in which an illuminating optical system and observing optical system are coaxially arranged so as to eliminate the unevenness of the illumination caused by the noncoincidence of the ranges A.sub.1 and A.sub.2 with each other. A half-prism 8 is inserted in the objective 5 so that the light bundle projected out of the light guide fibers 1 will enter the side surface of the half-prism 8, will then be reflected and will be projected toward the surface 3 of the object to be observed out of the cover glass 4 of the observing optical system. Therefore, in this case, even if the tip part 7 of the endoscope approaches the surface 3 of the object to be observed, the illuminated range A.sub.1 will be able to well cover the observed range A.sub.2. However, in this system, there is a defect that no clear image can be obtained due to such flare as of the surface of the cover glass 4. Next shown in FIG. 3 is a method wherein at least a pair of projections 9, 9 opposed to each other are provided in the tip part of the endoscope to keep constant the distance between the surface 3 of the object to be observed and the tip surface of the endoscope so that, at the time of the observation, these projections 9 will be in close contact with the surface 3 of the object to be observed. However, even in this method, in case the magnification of the objective 5 is more than twice as large, the focal depth will become so shallow that, if the distance to the surface of the object to be observed varies, the image will be likely to be out of focus. It has been inconvenient.